1. Field of the Invention
The present invention relates to an armature. More particularly, it relates to an armature core for the armature.
2. Discussion of Background
FIG. 2 is a front view of a typical armature used for a conventional d.c. motor; FIG. 3 is a cross-sectional view of the armature, and FIG. 4 is an enlarged cross-sectional view partly broken which shows slots formed in the conventional armature.
In these figures, a reference numeral 1 designates a motor shaft. A commutator 2 and an armature core 3 are respectively fitted to the motor shaft 2. An armature coil 4 is fitted in the armature core 3.
As shown in FIG. 4, the shape of the slot of the conventional armature core 3 is constructed in such a manner that the size of an opening portion 5a is made smaller than the diameter of a wire constituting the armature coil 4 to constrain the coil 4 during the revolution of the armature and a magnetic flux formed in association with a field (not shown) is uniform. Heretofore, there has been a demand of manufacturing a motor which allows the output power to be increased without changing the shape of the structural elements for the armature as shown in FIGS. 2-4. In this case, the cost of the armature required for model-changing can be reduced by increasing, for instance, only the diameter of the core and by using the other structural elements as they are. When such measures are taken, the shape of the slot is formed as in FIG. 5, for instance. In FIG. 5, the shape and the dimensions of a coil receiving portion 11b formed in the slot are the same as those of the portion 5b in FIG. 4, and the length of a throat portion formed between the opening portion 11a of the slot and the coil receiving portion 11b is changed from l.sub.1 to l.sub.2. In this case, the width b.sub.1 of the opening portion is not changed so that a magnetic flux formed in association with the field is made uniform.
However, the shape of the slot as shown in FIG. 5 has the following disadvantage. Namely, since the length of the throat portion of the slot is elongated from l.sub.1 to l.sub.2, the magnetic resistance of this area is lowered (the magnestic resistance is in proportion to the width b.sub.1 of the opening portion and is in inverse-proportion of the length l.sub.2 of the throat portion provided that the length of the laminated core sheets, i.e. the thickness of the core is unchanged), whereby there is produced a magnetic flux .PHI..sub.2 around the slot in addition to the main magnetic flux .PHI..sub.1 formed between the armature core and the field. Accordingly, the self-inductance of the coil becomes large to adversely affect current regulation, whereby reduction of output preperties is caused. Further, the intensity of a magnetic field formed in the outer space of the armature core 10 by a magnetomotive force caused by the aramature coil is reduced due to the magnetic flux .PHI..sub.2. This adversely affects the generation of torque.